Skin is typically characterized as consisting of three distinct layers, namely the stratum corneum, the epidermis and the dermis. The stratum corneum, the outermost layer, is made up of keratinized, cells, surrounded by intercellular space filled with lipids. The stratum corneum provides a substantial physical barrier to penetration of most substances to the lower layers of the skin. In addition to preventing transport of substances to the other skin layers, however, this barrier also aids in prevention of water loss from the skin. Both functions are primarily attributable to the presence of the lipids in the stratum corneum.
There are two sources of the skin surface lipids making up this important barrier; sebaceous glands and the epidermis. The lipids are a diverse group of compounds, comprising triglycerides, diglycerides, ceramides, free fatty acids, wax esters, cholesterol and cholesterol esters, and squalene. The quantity and composition of the skin surface lipids differ from place to place on the body, and may to some extent be related, to the number of sebaceous glands in a given area of the skin. The condition of the skin surface lipids may also be affected by an essential fatty acid deficiency. Additionally, the lipid barrier is easily diminished by exposure to harsh detergents or soaps, it is apparent, then, that the quality of the skin lipid barrier can vary widely, depending on a number of different factors, and therefore, may not always be adequate to perform its protective function optimally.
As an attempt to compensate for what may be a less than adequate lipid barrier, cosmetic compositions frequently incorporate components which compensate for water loss. Examples of such materials are hygroscopic humectants, e.g., urea or propylene glycol, which hold water on the skin; or emollients, e.g., oleyl alcohol or caprylic/capric triglycerides. Certain cosmetic components may be occlusive skin conditioners, which are used to provide an “artificial” barrier; such compounds are frequently lipids which remain on the skin surface, and include various hydrogenated oils, waxes and butters. Although many of these products provide an effective means of steaming water loss from the skin, they do have to be reapplied frequently to maintain the effect, and do not generally constitute a natural-occurring component of the stratum corneum, potentially giving rise to an unnatural, greasy feel to the skin. In addition, various pharmaceutical or cosmetic active agents are also frequently used to treat the symptoms of dry skin-associated conditions; however, in many cases, particularly with pharmaceutical agents, the treatments themselves may cause undesirable side effects in the individual being treated, while ultimately resulting in no actual repair of the lipid barrier.
The most desirable situation, from a functional point of view, is to find a way to enhance the skin's own ability to maintain and/or repair the strength of its barrier, so that the protective barrier formed is completely natural. It has now been discovered that a combination of specific skin active agents results in an unexpected increase in the strength of the natural barrier, by stimulating the production and maintenance of the barrier's naturally occurring components. There is thus provided a new type of cosmetic or pharmaceutical composition which functions by enhancing the skin's own functions, resulting in a more natural means of preventing dry skin and other undesirable results of a deficient lipid, barrier.